There are documented examples of batch furnace solutions designed for executing vacuum carburizing processes, where numerous workpieces arranged over a flat tray are processed simultaneously, such arrangement being multiplied on anything between a few and around a dozen tray levels. Single-chamber furnaces with an integrated high-pressure gas quenching system (HPGQ) are used for this purpose, two-chamber furnaces with a separated HPGQ chamber, or solutions enabling cooling in quenching oil.
For the purpose of mass production, modular systems are manufactured with multiple process chambers for vacuum carburizing and a separated chamber for loading/unloading the workload to/from individual process chambers, including equipment for HPGQ or oil quenching. There are documented furnace constructions with in-line process chamber arrangement, or with a circular arrangement around the rotation axis of the above-described quenching chamber. Various mutations of modular systems are applied for industrial purposes, including those enabling placement of one process chamber on top of another, as presented in the patent description EP 1319724 B1. All those systems are characterised by volumetric method of workload quenching in circulating gas—e.g. nitrogen or helium under high pressure (HPGQ)—or in quenching oil, with non-uniform quenching of individual workpieces in different areas of the workload due to non-uniform and non-repeatable flow of the quenching medium through the workload volume, as well as due to non-uniform flow of the quenching medium along workpiece surfaces, which further translates into quenching stress and eventually undesirable deformations.
Compared with oil quenching, in this case gas cooling is characterized by a higher rate of statistical repeatability of deformations.
Patent description DE102009041041 B4, on the other hand, presents a modular system designed for direct carburizing and quenching of such workpieces as e.g. gears with limited dimensions, enabling fast gas heating and cooling with a potential to further reduce deformations and/or uniformity of those deformations within one workload as well as repeatability in successive workloads. According to this patent, heating chambers are installed in a vertical arrangement—from two to six in a single vacuum housing. Under this system, workpiece loading takes place at only one level, workpieces being arranged on the surface of one tray, preferably made of CFC composite. This enables very fast heating of workpieces exposed to good penetration (without screening) of radiation from the chamber heating system during the heating phase, which allows to reduce the time spent by workpieces at high temperature level, and to ensure safe (sufficiently short) process time spent by workpieces at the temperature of ca. 1050° C., in the range of faster grain growth. The furnaces are designed for carburizing with layer thickness up to ca. 0.6 mm, for example.
Gas quenching of workpieces arranged in a single layer allows to use the HPGQ method with high repeatability and consistency due to simpler construction of the cooling gas circulation system, with uniform and thorough gas flow onto the workpieces arranged on the tray surface. It is easier to achieve high consistency with proper flow speed, pressure and temperature in relation to the flow of the cooling gas through the volumetric workloads. Loading of the workpieces arranged in a single layer facilitates automation of workpiece loading and unloading operations, while the progress related to achieving reduction and repeatability of deformations allows to install the furnace in a machine tool system between machines for rough gear processing and machines for finishing operations, while eliminating the transportation of workpieces to organizationally separated quenching shops.
As regards gas carburizing technology, for challenging workpieces (where volumetric quenching in quenching oil leads to higher deformations) separate quenching of individual workpieces is applied in a quenching press, with cyclical feeding to the press by an operator usually supplied with a manipulator, or in mass production where industrial robots are used.
On the other hand, in the technology of quenching non-rigid bearing rings there are tests of installations for cyclical feeding of rings to the cooling matrix, enabling quenching with gas or compressed air, with a suitable inflow of the cooling medium through nozzles arranged in proper relation to cooled surfaces, with a suitable pressure, at speeds from 50 to 100 m/s, at the level of 10 mm from the surface, which guarantees achieving cooling speeds of e.g. 15° C./s—comparable with quenching oil—relevant for quenching steel rings made from 100Cr6 steel [HTM53(1998)2 “Fixturhartung von Walzlagerringen unter Verwendug von gasformigen Abschreckmedien”]. 
With reference to experiences relating to gas carburising technology—employing vacuum carburizing—attempts have been made to design furnaces for mass production of volumetric workloads, as described above, but featuring continuous flow of the workload through the furnace, its structure comprising functional chambers for: heating, vacuum carburizing, diffusion, pre-cooling before quenching, as well as a quenching chamber (e.g. oil quenching) with chamber separation as above, employing vacuum locks. Such systems have been described (among others) in patent descriptions EP 0735149 of 1996, EP 0828554 of 2004, EP 1482060 of 2004 and in technical literature from the turn of the 1990′s. Unfortunately those technologies did not gain high popularity, mainly due to the level of deformations, non-uniformity of those deformations within one workload and between workloads, as well as due to the difficulty in maintaining continuous operation of the system.
Notably, there have been attempts to construct a continuously operated furnace intended for carburizing and quenching of individual workpieces fed through successive furnace systems designed for heating, carburizing, diffusion, pre-cooling and quenching. By way of example, there are systems described in patent description U.S. Pat. No. 4,938,458 (A) of 1990 “Continuous ion-carburizing and quenching system” and patent description EP 0811697 (B1) of 1997 “Method and apparatus for carburizing, quenching and tempering”. Also at the turn of the 1990's, a continuous furnace structure was produced with workload feeding on rollers, divided into functional chambers (loading and unloading locks as well as heating, carburizing, diffusion and pre-cooling chambers) and HPGQ chambers, presented (among others) in the title page of HTM 2/2001 “Multichamber continuous furnaces . . . ”. A new feature of this construction is the possibility of installing systems in line with machining solutions.
Production of toothed gears always includes the phases of rough and detailed machining—usually in the soft condition—as well as the phase of finishing individual gears after thermal and chemical treatment. Hence the continuous flow of individual workpieces for further processing after machining. Assuming that the technology of vacuum carburizing with direct quenching offers the effect of repeatable limitation of deformations and/or their repeatability relevant for the shape of workpieces, there is a demand for continuous process of carburizing and hardening of individual gears during a cycle corresponding to the cycle of machining for rough processing before thermo-chemical processing and finishing. Assuming a continuous flow of workpieces, cyclical (continuous) purging of individual workpieces after rough processing does not pose any technical or economic challenges.